Indeed, we want all sub-block leaves of the tree not to be smaller
than a minimum size for the low-rank approximation of sub-blocks to be
favorable compared to a full-rank computation in terms of complexity.
It is suggested that we take \(n_{\text{min}} =32\)  in order to avoid the overhead caused by
recursions; see [1]}.
Eventually, we
collect all leaves of the tree and split them into two partitions:
